We elect our legislators, who then make our laws. As bills move through Congress they are assigned numbers such as HR 1311 (House of Representatives Bill Number 1131). At the start of each Congressional session the numbers begin renumbering, so the bill numbers are not unique beyond that session. However, once signed by the President a law is assigned a 'Public Law' number as well as a 'Statutes at Large' number. Both are unique to the law that was passed -- no other laws will have these numbers, so laws can be cited using these numbers, and often are.
Ex. PL 109-459 (the "Call Home Act of 2006")
is a bill passed during the 109th Congress to direct the Federal Communications Commission to make efforts to reduce telephone rates for Armed Forces personnel deployed overseas, so they can more readily remain in touch with their families. Its Statutes at Large number is 120 Stat. 3399.
The current U.S. Code contains the current laws of the United States enacted by Congress. If someone is guilty of a federal crime it means he/she has violated the U.S. Code.